The present invention relates to conduits (i.e., pipes, tubes, and the like) made of extruded semi-crystalline plastic material such as polyamides, polyolefins such as polyethylene of high and low density, polypropylene, and thermoplastic polyesters which are well known and have numerous applications.
One use is as rigid or flexible compressed air distribution ducts; the latter constituting, for instance, a portion of the brake system of road vehicles such as trucks, in which the flexibility and the possibility of shaping of semi-crystalline plastic materials are especially appreciated. A preferred family of these plastic materials is that of the polyamides and among the latter, polyamide 11.
Within the scope of optimization of the manufacturing conditions of these pipes by extrusion and particularly in order to improve the mechanical properties of the pipes, one has found a molecular orientation which is produced at the surface of the pipes as a result of the friction of the yet plastic pipe against the surface of the means used to shape the pipe to the dimensions desired.
On the macroscopic scale, damage to the "skin" or surface of the conduits, such as abrasions, scratches, and the like, can be the cause of an abnormal percentage of breaks during impact resistance tests in the cold state carried out on samples of conduits.
On the microscopic scale, by birefringence measurements carried out on small plane sections adjacent to the axis of the pipes, an orientation parallel to that axis was found of a thickness between 20 and 200 microns, and more precisely between 50 and 100, microns. One has been able to observe, furthermore, that the orientation maximum corresponding to the birefringence maximum was not located at the surface of the conduit, but at a depth of about twenty microns from the surface, measured in the direction of the axis of the conduit. What we will call the "crystalline morphology" of the conduit is characterized by the thickness of the oriented skin and its orientation level in absolute value.
By pursuing a theory put forth by Dragaun, Hubeny and Muschik in the Journal of Polymer Science, vol. 15, pages 1779 to 1789 (1977), it was thought that the orientation maximum probably corresponds to an inflection point at the rate profile established at a variable distance from the wall of the conduit or to a maximum of shearing velocity at these same points of measurement, when the pipe re-enters the shaping means used to form the conduit to the precise dimensions desired.